The diet and growth of fox cubs in two regions of Scotland

TL;DR

This study compares the diet and growth of fox cubs in North-east and West Scotland, finding regional differences in stomach contents and a later breeding season in the West. Growth rates were similar, but the growth curves were shifted, implying cubs in the West were born about one to two weeks later.

Key findings

• Stomach contents differed by region: in North-east Scotland lagomorphs occurred in 76% of main stomach contents and 87% of total contents, versus 6% and 19% in the West (p=0.001). In the West, lamb occurred in 38% of main and 41% of total contents versus 2% and 5% in the North-east (p=0.001), and *Microtus* occurred in 28% main/41% total versus 5%/16% (p=0.05).
• Growth rate of litters was similar in the two regions (slope comparisons: head and body length F=1.896, d.f. 1,120, n.s.; weight F=0.561, d.f. 1,127, n.s.), but curve elevation differed highly significantly (head and body length F=34.617, d.f. 1,121, p<0.00001; weight F=31.175, d.f. 1,128, p<0.00001), indicating a later breeding season in the West.
• Estimated mean birth dates from head-and-body length were 26 March in the North-east and 3 April in the West; from weight they were 16 March and 28 March, respectively. Independent estimates using conception dates plus a 53-day gestation gave 27 March for the North-east and 10 April for the West; litters were estimated to be born over 6–7 weeks.

Methods

• Foxes were shot by hunters and forestry service in 1971–1976 in two regions of Scotland; each was weighed and measured, and stomach contents were collected for analysis. Table 1 reports stomach-content percentages for North-east (n=38) and West (n=32) samples, compared with 2×2 contingency tables.
• Growth was analyzed from Gompertz conversion factors for mean head-and-body length and weight of litters regressed on date, with analysis of covariance comparing slopes and elevations between regions. Birth dates were estimated from growth-curve intercepts assuming birth length 150 mm and weight 100 g; breeding-season limits used extrapolation of extreme litter values with average growth and only the lower half of each data set.

Limitations

• The paper states that conclusions about breeding dates and age of weaning depend on assumptions of conformity to a particular growth model, a standard weight and length at birth, and growth rate in all litters resembling the population average. The weight curve is described as nearer its lower tail and probably less reliable for extrapolation; breeding-season limits used only the lower half of each data set because of spread among older animals.
• Evident from the provided text: it is a partial excerpt, with methods details largely recoverable from the Polish summary and table headers, and some discussion of weaning is truncated.